N ov 1 99 7 X - ray Clusters at High Redshift

As the largest gravitationally bound structures known, clusters provide clear constraints on the formation of structure and on the composition of the universe. Despite their extreme importance for cos-mology the number of clusters at high redshift (z>0.75) is rather small. There are only a few X-ray emitting examples reported and a handful of optically-selected ones. These clusters can provide stringent constraints on theories of large scale structure formation, if they are massive enough. I will review the status of these distant X-ray selected clusters. These objects are of special importance because their X-ray emission and presence of gravitational arcs imply that they are massive, comparable to low redshift examples, and their existence is problematic for some theories of structure formation.